The present invention relates to a method of detecting adhesive material on a blank from which to form a container for tobacco articles.
In particular, the invention relates to a method of detecting adhesive material on a blank from which to form a hinged-lid packet of cigarettes, to which the following description refers purely by way of example.
Adhesive material is applied to cigarette packet blanks to join parts of the blank to one another, as well as to parts of a collar and to parts of a foil wrapping containing the cigarettes.
The adhesive material is applied by a gumming unit comprising a conveyor for feeding the blanks along a given path, and one or more gumming devices arranged along the path to apply the adhesive material to the blanks.
One commonly used type of gumming device comprises at least one gumming roller, which is positioned contacting adhesive material in a tank, and rolls along the blanks to deposit the adhesive material as the blanks are fed along the path.
Alternatively, adhesive material is applied by spray guns, which spray a quantity of adhesive material through nozzles as the blanks are fed past the guns.
Adhesive material is also applied to the blanks by means of spreading guns, which, like spray guns, dispense a controlled quantity of adhesive material through nozzles, but which, unlike spray gumming devices, is spread as opposed to being sprayed, onto the blanks.
The gumming unit provides for applying adhesive material to given portions of the blanks, and in sufficient quantities to ensure the packets formed from the blanks are stable.
The gumming devices described above do not always provide for applying the adhesive material correctly and in the right quantities to the given portions of the blanks. Which means the gumming must be checked to enable any improperly gummed blanks to be rejected immediately and prevented from being formed into packets, which will inevitably have to be rejected and opened to salvage the cigarettes inside. This is especially so when applying adhesive material using spray or spreading gun gumming units, the nozzles of which are subject to clogging, due to impurities in the adhesive material, or due to the adhesive material drying and hardening at the nozzle outlets.
One known method of eliminating the above drawbacks is to check the flow of adhesive material along a supply conduit common to a number of nozzles remains constant, or varies within predetermined limit values and in time with the passage of the blanks past the guns. Such a method, however, fails to meet current requirements, by failing to determine small variations in flow, and is especially inaccurate in the case of gumming units with a large number of nozzles.
Another known method is to check the flow of adhesive material through each nozzle by means of a sensor located along the conduit supplying the adhesive material to the nozzle or at the outlet of each nozzle. Though more effective, this solution complicates the design of the guns by requiring a flow sensor for each nozzle.
A further drawback of the above methods is that they fail to provide for determining whether the adhesive material is applied to the correct portion of the blank. That is, even if the flow of adhesive material indicates correct operation of the guns, there is no guarantee that the adhesive material has been applied at the right points. Moreover, methods based on measuring the flow of adhesive material cannot be applied when depositing adhesive material using the gumming rollers described previously.
From GB-A-2297616 it is known to check the masses of adhesive material applied to the blank in order to find whether the masses of adhesive material are applied in the correct position and cover a given length. According to the method referred above, a check is made in a comparison device between a predetermined linear pattern and a detected linear pattern, in case the two patterns do not coincide, the comparison device transmits an error signal.
This method, even though it has proved to be more reliable than the methods previously described, has the drawback that the comparison device, which acts on a strict coincidence between the predetermined pattern and the detected pattern, produces an error signal even when the masses adhesive material contain a sufficient quantity of adhesive material and are arranged in an acceptable position.
Moreover, the detection of glue masses is based on the difference between peaks of darkness and peaks of brightness, which are generated by inclined light beams impinging upon the blank and the glue masses. The glue masses are offset in respect of the flat blank and, for this reason generates shadows, which correspond to the peaks of darkness, and reflect part of the beams generating in this way the peaks of brightness. A blank from which to form a container for tobacco articles is provided with prescored lines along which the blank is folded. The prescored lines are offset in respect of the level of a flat blank in the same way as the masses of adhesive material applied on the blank itself, then the check method described in GB-A-2297616 is not suitable for checking the presence of glue on blanks from which to form a container for tobacco articles, because of the prescored lines.